In recent years, enhancement of fuel efficiency of automobiles has become an important issue from the viewpoint of global environment protection. Consequently, there is an active movement to reduce the thickness of vehicle body components through increases in strength of vehicle body materials, and thereby reduce the weight of the vehicle body itself.
In general, to strengthen a steel sheet, it is necessary to raise the proportion of a hard phase such as martensite or bainite relative to the entire microstructure of the steel sheet. However, strengthening a steel sheet by raising the proportion of a hard phase leads to degradation in formability. Therefore, it has been desired to develop a steel sheet that has both high strength and excellent formability. To date, various multi-phase steel sheets have been developed such as ferrite-martensite dual phase steel (DP steel) or TRIP steel utilizing transformation-induced plasticity of retained austenite.
If the proportion of hard phase is raised in a multi-phase steel sheet, the formability of the steel sheet will be strongly affected by the workability of the hard phase. This is because if the proportion of the hard phase is low and there is a large amount of soft polygonal ferrite, then deformability of the polygonal ferrite will be dominant over formability of the steel sheet. Therefore, formability of the steel sheet such as ductility can be ensured even if workability of the hard phase is not enough. On the other hand, if the proportion of hard phase is high, deformability of the hard phase itself, rather than deformability of the polygonal ferrite directly affects the formability of the steel sheet.
Thus, in the case of a cold-rolled steel sheet, it is subjected to heat treatment to control the amount of polygonal ferrite generated during annealing and subsequent quenching processes. The steel sheet is then subjected to water quenching to generate martensite, which is tempered by reheating and retaining the steel sheet at a high temperature so that carbides are generated in the martensite of hard phase to improve workability of the martensite. However, such quenching and tempering of the martensite require special production facilities such as, e.g., continuous annealing facilities with the ability of water quenching. Accordingly, in normal production facilities without the ability of subjecting a steel sheet to water quenching and then reheating and retaining it at high temperature, it is indeed possible to strengthen the steel sheet, but it is not possible to improve the workability of martensite as the hard phase.
In addition, as an example of a steel sheet having a hard phase other than martensite, there is a steel sheet in which a primary phase is polygonal ferrite and a hard phase is bainite and pearlite, and carbides are generated in such bainite and pearlite serving as the hard phase. This steel sheet exhibits improved workability not only by polygonal ferrite, but also by generating carbides in the hard phase to improve workability of the hard phase in itself, where, in particular, an improvement of the stretch-flangeability is intended. However, since the primary phase is polygonal ferrite, it is difficult to achieve both an increase in strength to 780 MPa or more in terms of tensile strength (TS) and formability. In this connection, even when workability of the hard phase itself is improved by generating carbides in the hard phase, the level of workability is inferior to that of polygonal ferrite. Therefore, if the amount of polygonal ferrite is reduced to increase the strength to 780 MPa or more in terms of tensile strength (TS), adequate formability cannot be obtained.
To address the above-described problem, for example, JP 4-235253 A proposes a high strength steel sheet having excellent bendability and impact properties, wherein alloy components are specified and the steel microstructure is fine uniform bainite including retained austenite.
JP 2004-076114 A proposes a multi-phase steel sheet having excellent bake hardenability, wherein predetermined alloy components are specified, the steel microstructure is bainite including retained austenite, and the amount of retained austenite in the bainite is specified.
JP 11-256273 A discloses a multi-phase steel sheet having excellent impact resistance, wherein predetermined alloy components are specified, the steel microstructure is specified such that bainite including retained austenite is 90% or more in terms of area ratio and the amount of austenite in the bainite is 1% or more and 15% or less, and the hardness (HV) of the bainite is specified.
JP 2010-090475 A proposes a high strength steel sheet having excellent formability, wherein a predetermined alloy composition and a predetermined steel microstructure are specified, adequate strength is ensured by a martensite phase, stable retained austenite is ensured by upper bainite transformation and, furthermore, a part of the martensite phase is tempered martensite.
Hereafter, an important challenge to achieve even wider application of high strength steel sheets, in particular, steel sheets in 780 MPa grade or higher of strength, is how to improve ductility and/or bendability when enhancing the strength of steel sheets, while preserving the absolute value of stretch flangeability. Relating to this problem, however, the above-mentioned steel sheets are facing the following problem.
That is, the steel disclosed in JP 4-235253 A indeed has excellent bendability, but in most cases does not provide sufficient stretch flangeability, which limits its application range.
In addition, while the steels disclosed in JP 2004-076114 A and JP 11-256273 A have excellent impact absorption ability, no consideration is given to stretch flangeability at all, which limits the application of these steels to those parts requiring stretch flangeability during forming, and as a result, these steels are applicable in a limited range.
The steel sheet disclosed in JP 2010-090475 A addresses the above-described problem by using the microstructure of steel without ferrite. That steel sheet has excellent stretch flangeability and ductility depending on the strength level, in particular, when it is required to have a strength of 1400 MPa or more. However, it cannot be said that that steel sheet ensures sufficiently high stretch flangeability required for the material at the strength level of less than 1400 MPa, which also limits the application of this steel sheet.
It could therefore be helpful to provide a high strength steel sheet having excellent formability, in particular, ductility and stretch flangeability, and having a tensile strength (TS) of 780 MPa or more, and an advantageous method of manufacturing the same.
It should be noted that examples of high strength steel sheets include steel sheets in which hot-dip galvanizing or galvannealing is applied to a surface of the steel sheet.
In addition, as used herein, the term “excellent formability” indicates that the following conditions are met: λ value, which is an index of stretch flangeability, is 25% or more regardless of the strength of the steel sheet, and a product of TS (tensile strength) and T.EL (total elongation), or the value of TS×T.EL is 27000 MPa·% or more.